Players don't place a certain number of tokens each round; they place tokens until everyone only has one token left behind their screen. The First Player card doesn't change that.

What you may be misunderstanding is that the "Token Pool" to which the token is returned is not the same as the player's hand of tokens. Rather, it is the pool of face-down tokens from which the player draws his tokens.

Since this thread is about the first player card. I take it that it can be used at any point during the placement phase as long as it is done before the first player places a token. It doesn't have to be the first thing done by the first player at the start of the placement phase.

In regards to playing the First player card and you happen to choose a bluff token, does this return to the owner's Pool or Hand?

The rulebook clearly states discarded bluff tokens are always returned to their owners’ hands. But as the First player card is written it isn't really a discard.

Would they effectively "lose" their bluff token until they potentially drew it from their pool in a future upkeep phase?

Official answer from FFG:

"That’s a great question. If you choose the bluff token, it is returned to its owner’s hand, but set aside until the next round; they cannot use it this round. I would recommend having them place it faceup in front of their daimyo screen for the duration of that round to ensure they don’t accidentally use it."